More than 400 freshly isolated oral bacteria obtained from subgingival samples of human subjects were identified and analyzed for their ability to coaggregate with other previously characterized oral actinomycetes and streptococci (stock culture collection strains - some isolated more than 10 years ago). The coaggregation properties of the new and old strains were compared. Of the new isolates, the predominant participants were Actinomyces viscosus, A. naeslundii, and Steptococcus sanguis. All of the A. viscosus and more than 70% of the A. naeslundii and S. sanguis isolates coaggregated. However, none of a wide variety of other common oral bacteria, including S. mutans, coaggregated. Using the new isolates, four new patterns of coaggregation were observed; however, more than 80% of the patterns with both new and old strains were identical. These results indicate that the cell surface components which are involved in these coaggregations are very stable structures and that they mediate a highly specific set of cell-cell interactions. Further study of these interactions between oral bacteria is expected to provide a better understanding of the ecology of these bacteria in the subgingival ecological niche.